1. Field of the Invention
The present invention relates to a color image forming apparatus by which a color image can be read and can be reproduced by using a plurality of kinds of developing agent, and more particularly relates to a color image forming apparatus in which in the case that the read color image is composed of and expressed only with colors of one or two kinds of developing agent the developing operation with the other remaining developing agents can be excluded.
2. Description of the Prior Art
It is general that a color copying machine which is one of the examples of such a color image forming apparatus as shown above is what is provided with four developers, each of which accommodates and corresponds to respective toners (developing agents) of the three colors of YELLOW (Y), MAGENTA (M), and CYAN (C) and BLACK (Bk). The color image forming apparatus can resolve a color image read by for example a CCD sensor to the three primary colors (RGB) of light and makes output per primary color. Furthermore, the color image forming apparatus repeats developing operations in order per developer accommodating per kind of toner in three or four kinds of them (Y, M, C, or Y, M, C, Bk) according to the output per primary color coming from the CCD sensor, and reproduces the above color image onto a transfer sheet.
In such a color copying machine described above, there is such a type as having a function to judge whether or not the color image of, for example, a document is BLACK through executing preparatory reading scanning for the document. For example, when the R output, G output and B output obtained through the preparatory reading scanning by the CCD sensor are almost the same value (there are many cases that the output is zero) or the R output, G output and B output are those of a specified ratio which is peculiar to BLACK according to the spectral characteristics of the light source, the color copying machine judges that the color image is only of BLACK (Bk) and executes the developing operation only with a developer which accommodates the BLACK toner, thereby causing the developing operations with the developers accommodating the other toners than BLACK to be excluded.
Hence, in the case that the color image is only of a color (for example, Y, M or C) possible to be expressed only with for example one kind of color toner or only a color (for example, RED (R), GREEN (G), or BLUE (B)) possible to be expressed with two kinds of color toner except BLACK toner in each case, it should be possible to do with the developing operations only with one or two kinds of toner corresponding thereto.
However, even though the color image is of only the color possible to be expressed with the above one or two kinds of toner on the color except BLACK, the conventional color copying machine has repeatedly executed the developing operations by using all developers of toners of respective colors Y, M and C, according to each output of R, G and B coming from the CCD sensor.